1. Field of the Invention
The present invention relates to an optical recording and/or reproducing apparatus operative to produce a plurality of light beams each divided into at least a main light beam and an auxiliary light beam and to cause the main and auxiliary light beams to impinge upon a disc-shaped record medium for recording information thereon or reproducing information therefrom and producing a tracking error signal.
2. Discussion of Prior Art
In an optical disc player for optically recording an information signal on or optically reproducing an information signal from an optical disc-shaped record medium, an optical head device is provided for constituting an optical arrangement for causing a light beam to impinge upon the optical disc-shaped record medium so as to form record tracks corresponding to a recording information signal on the optical disc-shaped record medium or to read an information signal recorded in the record tracks on the optical disc-shaped record medium.
In the optical head device, for example, a laser light beam produced by a light beam generating portion is collimated by a collimator lens and enters into an objective lens to be focused thereby to impinge upon the optical disc-shaped record medium, and a reflected light beam emanating from the optical disc-shaped record medium is directed through the objective lens to a beam splitter to be changed in a direction of its optical axis thereby so as to enter a photodetecting portion. Then, a reproduced information signal, a focus error signal and a tracking error signal are produced based on a detection output of the reflected light beam from the photodetecting portion.
For the production of the tracking error signal, various systems including the so-called "Push-Pull system" have been known. In the Push-Pull system, the reflected light beam from the optical disc-shaped record medium is detected by a photosensor having a photodetecting element divided into two parts, and the tracking error signal is produced based on a difference in level between detection outputs obtained from the two parts of the photodetecting elements, respectively. In the case where the tracking error signal is produced in accordance with the Push-Pull system, although an optical arrangement for obtaining the tracking error signal and an associated signal processing circuit arrangement can be relatively simple, there is the disadvantage that a light beam spot formed on the photosensor by the reflected light beam from the optical disc-shaped record medium is undesirably moved regardless of the tracking condition of the light beam impinging upon the optical disc-shaped record medium, and therefore the tracking error signal contains undesirable DC offsets when the optical disc-shaped record medium is inclined in its radial direction to the optical axis of the objective lens through which the light beam is incident upon the optical disc-shaped record medium.
Accordingly, with the intention of avoiding the above mentioned disadvantage of the Push-Pull system, there has been proposed an improved Push-Pull system as disclosed in the Japanese patent application published before examination with laid-open number 61-94246. In this case, at least two light beams which are obtained by dividing a light beam produced by a light beam source are caused to impinge upon an optical disc-shaped record medium for forming respective light beam spots thereon with a space therebetween in the radial direction of the optical disc-shaped record medium, which corresponds to a distance in a predetermined relation to the track pitch of the spiral record track. Then, each of two reflected light beams from the optical disc-shaped record medium is detected by a respective photosensor having a photodetecting element divided into two parts, and a difference in level between detection outputs obtained respectively from the two parts of one photodetecting element is subjected to a level adjustment for compensating a difference in strength between the two reflected light beams and then subtracted from a difference in level between detection outputs obtained respectively from the two parts of the other photodetecting element, thereby to produce the tracking error signal.
The tracking error signal thus obtained is prevented from containing DC offsets, so as properly to represent the tracking condition of the light beam impinging upon the optical disc-shaped record medium, even if it is inclined in its radial direction to the optical axis of the objective lens.
In addition to an ordinary optical disc player in which a single light beam is produced to impinge upon a disc-shaped record medium directly or after being divided into a plurality of light beams for recording information in or reproducing information from each record track on the disc-shaped record medium, the so-called "Multichannel Type" optical disc player has been also proposed. In the Multichannel Type optical disc player, a plurality of light beams are produced simultaneously so that each of the light beams produced simultaneously is caused to impinge upon a disc-shaped record medium directly or after being divided into a plurality of light beams for recording information in or reproducing information from a respective record track on the disc-shaped record medium. To such a Multichannel Type optical disc player, it is also desirable to apply the improved Push-Pull system for producing a tracking error signal in view of the advantage that a tracking error signal superior in quality can be obtained with simplified optical and circuit arrangements.
The previously proposed improved Push-Pull system, however, has been restricted to being applied to the optical disc player in which the single light beam is produced to impinge upon the disc-shaped record medium directly or after being divided into a plurality of light beams for recording information in or reproducing information from each record track on the disc-shaped record medium, and therefore no Multichannel optical disc player employing the improved Push-Pull system has been found.